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The solutions for exercise 03&04 of MIT.Missing-semester(2020)

MartinLwx  included in  category Course
     1450 words   7 minutes 

Complete vimtutor. Note: it looks best in a 80x24 (80 columns by 24 lines) terminal window.

It is a tutorial for beginners of vim. I will just put some notes which are not mentioned in course here.

  • U command: When we press u in normal mode, we can undo the last command. What U does is fixing a whole line.

  • Ctrl + G: show your location in the file and the file status. Type the linenumber you want to go, then press G, then you are there.

    • To search for a phrase in the backward direction, use ? instead of / .

  • Type :! followed by an external command to execute that command.

  • Select text to write

    • Use visual mode to select text
    • type :w <type_filename_here
    • You can also type :!ls to verify this

  • To insert the contents of a file, type :r FILENAME

    • Furthermore, You can also read the output of an external command. For example, :r !ls reads the output of the ls command

  • Type a capital R to replace more than one character.

Download our basic vimrc and save it to ~/.vimrc. Read through the well-commented file (using Vim!), and observe how Vim looks and behaves slightly differently with the new config.

I would recommend making your own configuration.

Install and configure a plugin: ctrlp.vim

  1. Create the plugins directory with mkdir -p ~/.vim/pack/vendor/start
  2. Download the plugin: cd ~/.vim/pack/vendor/start; git clone https://github.com/ctrlpvim/ctrlp.vim
  3. Read the documentation for the plugin. Try using CtrlP to locate a file by navigating to a project directory, opening Vim, and using the Vim command-line to start :CtrlP.
  4. Customize CtrlP by adding configuration to your ~/.vimrc to open CtrlP by pressing Ctrl-P.

PASS

To practice using Vim, re-do the Demo from lecture on your own machine.

PASS

Use Vim for all your text editing for the next month. Whenever something seems inefficient, or when you think “there must be a better way”, try Googling it, there probably is. If you get stuck, come to office hours or send us an email.

Configure your other tools to use Vim bindings (see instructions above).

I have already enable vim mode in my Vscode and zsh. 💪

Further customize your ~/.vimrc and install more plugins.

I have made my own configuration

(Advanced) Convert XML to JSON (example file) using Vim macros. Try to do this on your own, but you can look at the macros section above if you get stuck.

The steps:

  1. Press Gdd && ggdd to delete the first line and the last line
  2. Macro to format a single element (register e)
    • Go to line with <name>
    • qe^r"f>s": "<ESC>f<C"<ESC>q
  3. Macro to format a person
    • Go to line with <person>
    • qpS{<ESC>j@eA,<ESC>j@ejS},<ESC>q
  4. Macro to format a person and go to the next person
    • Go to line with <person>
    • qq@pjq
  5. Execute macro until end of file
    • 999@q
  6. Manually remove last , and add [ and ] delimiters

The solution above is provided by the official course site.

Take this short interactive regex tutorial.

Just click this link to finish this regex tutorial.

Find the number of words (in /usr/share/dict/words) that contain at least three as and don’t have a 's ending. What are the three most common last two letters of those words? sed’s y command, or the tr program, may help you with case insensitivity. How many of those two-letter combinations are there? And for a challenge: which combinations do not occur?

The answers of questions in exercise are ⬇️

Q: Find the number of words (in /usr/share/dict/words) that contain at least three as and don’t have a 's ending.

Answer: tr 'A-Z' 'a-z' < /usr/share/dict/words | grep -E '.*a.*a.*a.*[^s]$' | wc -l, 👉 5290

  1. Use tr 'A-Z' 'a-z' < /usr/share/dict/words to make text case-insensitive

  2. Use grep -E 'grep -E '.*a.*a.*a.*[^s]$' to find the words that contain at least three a and don’t have a 's ending

    • The combination of .* means any character repeats any times.

    • [s] will match the s character. We add a ^ in [], which mean we want to match any single character excepet s

  3. Use wc -l to count the number of lines in output.

Q: What are the three most common last two letters of those words?

A: tr 'A-Z' 'a-z' < /usr/share/dict/words | grep -E '.*a.*a.*a.*[^s]$' | grep -E -o '.{2}$' | sort | uniq -c | sort | tail -n 1, 👉 1039 al and 763 an and 637 ae

  1. Use grep -E -o '.{2}$' to get last 2 letters of these words
    • -o means Prints only the matching part of the lines. In this case, what we want is the last 2 letters, so we type .{2}$
  2. Use sort | uniq -c to get the two-letter combinations count
    • This can ensure the combinations are uniq.
  3. Use sort | tail -n 3 to sort previous results according to their frequency counts

Q: How many of those two-letter combinations are there?

A: tr 'A-Z' 'a-z' < /usr/share/dict/words | grep -E '.*a.*a.*a.*[^s]$' | grep -E -o '.{2}$' | sort | uniq -c | wc -l, 👉 140

Q: And for a challenge: which combinations do not occur?

diff <(echo {a..z}{a..z} | tr " " "\n") \
		 <(tr 'A-Z' 'a-z' < /usr/share/dict/words | grep -E '.*a.*a.*a.*[^s]$' | grep -E -o '.{2}$' | sort | uniq -c | sort | awk '{print $2}' | sort) \
		 | grep -E "<" \
		 | wc -l
# output: 536
  1. Use echo {a..z}{a..z} to get all two-letter combinations. However, in order to compare 2 sets of combinations(this one && Our previous results), we need to use \n as delimiter of each combination. We can use tr " " "\n".
  2. In the previous question, we can get every different combinations and their frequency counts. Each row looks like <frequency count> combination. In order to get the combinations, we can use awk {print $2}, $2 means the second field in each row. After that, we need to
  3. Then we need a tool to compare the 2 sets of combinations. Here comes the diff command. diff will compare 2 files line by line. We also need Process substation to pass the 2 sets of combinations as arguments of diff

To do in-place substitution it is quite tempting to do something like sed s/REGEX/SUBSTITUTION/ input.txt > input.txt. However this is a bad idea, why? Is this particular to sed? Use man sed to find out how to accomplish this.

This exercie remind me of the shellcheck tool. So I just type sed s/REGEX/SUBSTITUTION/ input.txt > input.txt in a test.sh file and run shellcheck test.sh. Then I knew THIS IS A BAD IDEA. 📒 We should not read and write the same file in the same pipeline. After checking man sed carefully, I found 2 flags helpful–-i and -I. Both of them can edit file in-place. More information, you may check

Find your average, median, and max system boot time over the last ten boots. Use

journalctl

on Linux and

log show

on macOS, and look for log timestamps near the beginning and end of each boot. On Linux, they may look something like:

Logs begin at ...

and

systemd[577]: Startup finished in ...

On macOS, look for:

=== system boot:

and

Previous shutdown cause: 5

I am a macos user. I barely shutdown my Macbook Pro. So when I ran log show | grep -E "log show | grep -E "system boot", it kept running like it will never stop 😿. So I decided to skip this exercise.

Look for boot messages that are not shared between your past three reboots (see journalctl’s -b flag). Break this task down into multiple steps. First, find a way to get just the logs from the past three boots. There may be an applicable flag on the tool you use to extract the boot logs, or you can use sed '0,/STRING/d' to remove all lines previous to one that matches STRING. Next, remove any parts of the line that always varies (like the timestamp). Then, de-duplicate the input lines and keep a count of each one (uniq is your friend). And finally, eliminate any line whose count is 3 (since it was shared among all the boots).

PASS 😿

Find an online data set like this one, this one, or maybe one from here. Fetch it using curl and extract out just two columns of numerical data. If you’re fetching HTML data, pup might be helpful. For JSON data, try jq. Find the min and max of one column in a single command, and the difference of the sum of each column in another.

PASS 😿

  1. Accessing last x characters of a string in Bash

  2. Process substitution

Updated on 2021-12-26
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